On Fri, Dec 12, 2008 at 10:29:29AM -0800, Scott Lurndal wrote: > On Fri, Dec 12, 2008 at 05:02:05PM +0100, Gerd Hoffmann wrote: > > Matthew Wilcox wrote: > > > On the other hand, NetBSD have approximately 0% market share. > > > We shouldn't let them lock us into making a bad decision. Is there > > > anyone other than NetBSD who has added these syscalls? > > > > Free- and OpenBSD have it too. For Solaris I've found a feature request > > only. Dunno about MacOS/Darwin. Other un*xes which are important these > > days? > > > > I'd *really* hate it to have the same system call with different > > argument ordering on different systems though. Especially when swapping > > two integer values, so gcc wouldn't error out on wrong usage. > > I would suggest that from the end-users perspective, the user-mode API > should be similar to pread/pwrite, e.g: > > int preadv(fd, iovec, iovec_size, offset) Yes, and that's easy for glibc to achieve. What's hard is that the user <-> kernel API firstly has a limited number of registers available to it for passing arguments without indirection from user space into kernel space. Secondly, the user <-> kernel argument register allocation can vary depending on the ABI version which user space or kernel space is built for. On ARM we have two ABIs, one where 64-bit arguments can be placed in any two consecutive registers, and one where 64-bit arguments must be placed in an even,odd register pair (not an odd,even pair.) That leads to the above being: fd r0 r0 iovec r1 r1 vecsz r2 r2 offset r3,r4 r4,r5 Notice the different register allocation for the 64-bit offset. This problem of register-aligned argument placement is not limited to just ARM, but several other Linux supported architectures. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
